Polymer particles, metal particles, and the like have been used in various applications. Although the following is merely one example of the applications, polymer particles are used in fiber applications. In such applications, particles having sizes on the orders of μm and nm, are expected to exhibit new properties. Furthermore, metal particles having particle diameters on the order of nm, are called metal nanoparticles. Such metal nanoparticles have melting points dramatically lower than the melting point of the bulk substance. Accordingly, the applications of metal nanoparticles to conductive pastes and the like, which need to be melted at low temperatures, are expected, although this is merely one example of the applications. Therefore, it is required to control properties of polymer particles, metal particles, and the like according to the applications.
In addition, there are generally two types of method for producing metal nanoparticles: a breakdown method based on physical processing; and a buildup method based on chemical processing. Of these, the buildup method does not require as large dedicated machines as those for the breakdown method, and accordingly, it is widely employed. As a buildup method, a method is known in which metal ions are chemically reduced in a solvent.
In relation to this method, numerous methods for synthesizing silver nanoparticles in aqueous solutions, for example, have been studied. Typically, the Carey Lea method has been studied in which an aqueous solution of silver nitrate is added into an aqueous solution of a ferrous salt and a citric acid salt. By these methods, a liquid dispersion containing silver nanoparticles having particle diameters on the order of 10 nm, can be obtained. Such a liquid dispersion is excellent in high dispersion stability and narrow particle size distribution. Furthermore, it is known that properties of liquid dispersions of metal nanoparticles, such as a liquid dispersion of silver nanoparticles and the like, are greatly changed by controlling the particle diameters, particle diameter distribution, shapes, and the like of the metal nanoparticles.
One example of the method for producing such metal nanoparticles includes a production method in which: to control the shapes and particle diameters of silver nanoparticles (silver powder), a slurry containing an ammine complex of a silver salt and an ammine complex of a heavy metal salt acting as a habit modifier during a reduction reaction, is mixed at once with a solution containing potassium sulfite used as a reducing agent and a gelatin used as a protective colloid, the ammine complex of the silver salt is reduced so as to synthesize silver nanoparticles, and these silver nanoparticles are then recovered. (See, for example, Patent Document 1.)
In addition, in the production process of metal nanoparticles, it is necessary to suppress the self-assembly of metal nanoparticles synthesized by the reduction reaction so that the sizes of the metal nanoparticles are decreased. The measure of decreasing the sizes of metal nanoparticles includes: decreasing the concentration of a reaction solution; decreasing the volume of a reaction solution to be mixed; and the like. However, in the case of decreasing the concentration of a reaction solution, waste liquids are generated in a large amount, and a large amount of energy for the condensation is required. These bring about a problem that the environmental burden is increased. Therefore, the measure of decreasing the sizes of metal nanoparticles is preferably decreasing the volume of a reaction solution to be mixed. Furthermore, when producing metal nanoparticles, it is necessary to speed up the reduction reaction and quickly stabilize the reaction by a dispersant or the like.
Thus, as one example of the method for synthesizing a liquid dispersion of metal nanoparticles, to suppress the self-assembly and to speed up the reduction reaction, a method for synthesizing a liquid dispersion of gold nanoparticles has been proposed in which chloroauric acid (HAuCl4) solution is mixed with a reducing agent (such as sodium borohydride, citric acid, ascorbic acid, or the like) by using a microreactor (micromixer) so that gold ions are reduced and gold atoms are then formed. The microreactor used in the synthesis method is configured such that liquids pass through multiple tubular flow paths and then merge for the mixing. Such a microreactor makes it possible to decrease the volume of a reaction solution to be mixed, and also makes it possible to increase the mixing speed. Accordingly, the microreactor enables highly efficient mixing and high-speed reduction reaction. In addition, adding a dispersant into the microreactor can suppress the self-assembly of the gold atoms. (See, for example, Non-Patent Document 1.)
Furthermore, another example of the synthesis method to precisely control properties of metal nanoparticles and to efficiently synthesize the metal nanoparticles includes a method for synthesizing a liquid dispersion of metal nanoparticles as follows. Specifically, while a strong electric field is generated between two electrospray nozzles which are disposed opposite to each other in air by applying positive and negative potentials, respectively, to the two electrospray nozzles, solutions of a metal salt and a reducing agent are respectively supplied to the two electrospray nozzles at constant flow rates, and droplets respectively charged to the positive and negative potentials are sprayed from the electrospray nozzles so that these droplets collide and mix with each other in air by an electrostatic interaction. (See, for example, Patent Document 2.)